There has so far been widely used an inorganic electrophotographic photoreceptor having a photoreceptive layer containing the main component of inorganic photoconductive compounds such as selenium, zinc oxide, cadmium sulfide and silicon. However, these compounds are not necessarily satisfactory in sensitivity, heat stability, moisture resistance and durability. The photoreceptor comprising selenium, for example, lowers in photoreceptive properties due to crystallization, which makes its manufacture troublesome. Further, heat and fingerprints cause crystallization of selenium to deteriorate its photoreceptive properties. Cadmium sulfide has also problems in moisture resistance and durability, and zinc oxide in durability.
In order to solve these problems involved in inorganic photoreceptors, organic photoreceptors have been actively researched and developed in recent years, in which organic photoconductive compounds are incorporated into a photoreceptive layer as the principal ingredient. Japanese Patent Examined Publication No. 10496/1975 discloses an organic photoreceptor containing poly-N-vinylcarbazole and 2,4,7-trinitro-9-fluorenone, but this photoreceptor is not necessarily satisfactory in sensitivity and durability.
For the purpose of solving these problems, the development of an organic photoreceptor having higher quality has been attempted, in which carrier generation and carrier transport functions are separately provided by different substances. Such function-separating photoreceptors have advantages that the materials for respective functions can be selected from a wide range of compounds to thereby prepare photoreceptors of prescribed properties with ease. For this reason, various types of the function-separating photoreceptors have been studied, and some of them are already commercially available.
Multilayered photoreceptors of this type is described in U.S. Pat. No. 3,871,882, in which perylene derivatives are used for a carrier generation layer and oxadiazole derivatives for a carrier transport layer; and in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP O.P.I.) No. 84943/1980, in which distyrylbenzene-type bisazo compounds are used for a carrier generation layer and hydrazone compounds for a carrier transport layer.
These function-separating photoreceptors have various disadvantages as well as many advantages.
There have been proposed a variety of compounds as a carrier generation substance for the above photoreceptors, including amorphous selenium used in combination with organic photoconductive compounds as disclosed in Japanese Patent Examined Publication No. 16198/1968, while this method does not solve the problem that the carrier generation layer comprising amorphous selenium tends to crystallize on heating to thereby deteriorate in photoreceptive properties.
Further, there are proposed many photoreceptors in which organic dyes and organic pigments are used as carrier generation substances, including photoreceptors containing bisazo compounds as disclosed in JP O.P.I. No. 22834/1979, 73057/1980, 117151/1980 and 46237/1981, while these compounds do not fully meet various requirements in electrophotography because carrier transfer substances have to be limitedly selected for providing prescribed characteristics.